1. Technical Field
The present invention relates to an optical fiber base material manufacturing method and an optical fiber base material manufacturing apparatus, for manufacturing an optical fiber base material in an oxyhydrogen flame by means of hydrolysis particularly by using hydrogen obtained by vaporizing the liquid hydrogen.
2. Related Art
Hydrogen supply equipment that stores liquid hydrogen in a low-temperature reservoir, vaporizes it by raising its temperature, and supplies it to the equipment at the later stage has already been put to practical use. For example, such equipment is utilized for manufacturing highly pure quartz glass, which involves a VAD method or an OVD method. In both of these methods, silicide such as silicon tetrachloride (SiCl4) is supplied to an oxyhydrogen flame resulting from burning of hydrogen and oxygen by a burner, to generate silicon dioxide (SiO2) by means of hydrolysis, the result is deposited to generate a porous base material, and the porous base material is heated using an electric furnace thereby obtaining transparent highly pure quartz glass.
When manufacturing an optical fiber preform made up of a core high in refractive index and a cladding lower in refractive index than the core using a VAD method, germanium is often added to the core deposition burner as an additive so as to enhance the refractive index of the quartz glass. Germanium is added in the form of compound such as GeCl4. GeCl4 undergoes hydrolysis in the oxyhydrogen flame, to generate GeO2. SiCl4 or GeCl4, in the liquid form at normal temperature, is supplied either after vaporized by subjecting it to the carrier gas bubbling or after heated up to a temperature higher than the boiling point for direct vaporization.
Here, hydrogen supplied to a burner may be produced or stored at normal temperature, and liquid hydrogen may be used as backup hydrogen in case of stop of the normal hydrogen supply to a burner. There is also an alternative method in which all the hydrogen supply is made up of vaporized liquid hydrogen. In the manufacturing equipment adopting the VAD method or the OVD method, the flow rate of the gas to the burner is controlled using a mass flow controller (MFC).
In the VAD method, an optical fiber base material is manufactured by forming a porous base material by depositing glass particles generated in the flame hydrolysis on a rotating starting member, heating the porous base material in the heating furnace to about 1500 degrees centigrade to yield transparent quartz glass. During the depositing process, the deposition tip position of the porous base material is detected, for the purpose of adjusting the drawing up speed in accordance with the growth of the porous base material. Here, based on the finding that optical fiber base materials having a desirable refractive index distribution can be stably generated if the drawing up speed is maintained constant during the deposition process, the deviation from the set value of the drawing up speed is detected at each preset time interval, for correcting the flow rate of the raw material gas such as SiCl4 depending on the detected deviation, as disclosed in Patent Document No. 1. In Patent Document No. 2, for maintaining the drawing up speed constant, the hydrogen amount control is only performed on the cladding burner that is adjacent to the burner for the core, from among a plurality of cladding burners. Patent Document No. 1: Japanese Patent Application Publication No. H1-239033 Patent Document No. 2: Japanese Patent Application Publication No. H3-242341
When the VAD apparatus normally utilizing hydrogen produced at normal temperature has switched to hydrogen obtained by vaporizing liquid hydrogen when the normal hydrogen supply has stopped, the drawing up speed is unintentionally raised by about 2% and the core diameter is narrowed. Even when hydrogen obtained by vaporizing liquid hydrogen is usually used, the drawing up speed has changed according to the usage amount of the vaporized liquid hydrogen, to cause the core diameter to fluctuate. Patent Document No. 2 is a technology to perform hydrogen amount correction only to the burner adjacent to the core, so as to change the temperature of the core to change the drawing up speed as well as to correct the growing speed of the cladding, and is not designed to take into consideration the actual flow amount for both of these different origins of hydrogen. In Patent Document No. 2, normal gaseous hydrogen is used, and not the hydrogen obtained by vaporizing liquid hydrogen. Therefore, when using the hydrogen obtained by vaporizing liquid hydrogen instead, in the technology of Patent Document No. 2, the cladding growing speed substantially fluctuates even if the drawing up speed is maintained stable, to cause a problem of being unable to maintain the thickness of the cladding stable.
The change in flow rate of the hydrogen gas has a remarkable adverse effect on the optical characteristics of the resulting optical fiber base material manufactured in the VAD method, preventing stable generation of optical fiber base materials having a desirable refractive index distribution and leading to increase in ratio of defective products.